The Collaborative International Dictionary of English v.0.48:

Vanguard \Van"guard`\, n. [For vantguard, avantguard, F.
   avant-garde; avant before, fore + garde guard. See Avant,
   Ab-,Ante-, and Guard, and cf. Advance, Vamp, Van
   of an army, Vaward.] (Mil.)
   The troops who march in front of an army; the advance guard;
   the van.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vanguard
    n 1: the leading units moving at the head of an army [syn:
         vanguard, van]
    2: any creative group active in the innovation and application
       of new concepts and techniques in a given field (especially
       in the arts) [syn: avant-garde, vanguard, van, new
       wave]
    3: the position of greatest importance or advancement; the
       leading position in any movement or field; "the Cotswolds
       were once at the forefront of woollen manufacturing in
       England"; "the idea of motion was always to the forefront of
       his mind and central to his philosophy" [syn: vanguard,
       forefront, cutting edge]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "vanguard":
   advance guard, airhead, ancestor, announcer, antecedent,
   armed guard, avant-garde, bank guard, battle line, beachhead,
   bellwether, bridgehead, buccinator, bushwhacker, coast guard,
   cordon, cordon sanitaire, explorer, farthest outpost, first line,
   forebear, forefront, foregoer, forerunner, front, front line,
   front rank, front runner, front-runner, frontiersman, fugleman,
   garrison, goalie, goalkeeper, goaltender, groundbreaker, guard,
   guarder, guardsman, guide, harbinger, herald, inlying picket,
   innovation, innovator, jailer, latest fad, latest fashion,
   latest wrinkle, lead runner, leader, line, messenger, new look,
   newfangled device, novelty, outguard, outpost, pathfinder, picket,
   pioneer, point, precedent, precursor, predecessor, railhead,
   rear guard, scout, security guard, spearhead, stormy petrel,
   the in thing, the last word, the latest thing, trailblazer,
   trailbreaker, train guard, van, vaunt-courier, voortrekker,
   warder